The present invention relates to bulk superconductors and a novel seeding and growth process for preparing a bulk multi-domained superconducting material having strongly-linked low-angle domain boundaries.
Since the discovery of high temperature oxide superconducting materials having critical temperatures that exceed the temperature of liquid nitrogen, there has been a concerted effort to utilize these materials for various applications. Although many applications are aimed at replacing conventional superconductors in wires and electronic devices, new applications using bulk materials also have been proposed. These applications include, for example, use in energy storage devices such as flywheels, use in current leads for superconducting magnets, and in magnetic bearings, bulk magnets, and magnetic resonance imaging machines (MRI).
For applications such as those noted in the proceeding paragraph, high temperature superconducting materials with large critical current density (Jc) are required. One such high temperature superconducting material is a composite oxide of RE, Ba and Cu, and in particular, REBa2Cu3Ox (wherein RE representing at least one of the following rare earth elements: Y, La, Sm, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu).
One of the main factors that hinders the practical application of bulk high temperature REBa2Cu3Ox superconductors is the limited size in melt-processed material of the oriented domains, or regions which contain primarily small angle grain boundaries and consequentially have properties similar to those of single crystals. Additionally, the Jc of REBa2Cu3Ox has been restricted by the presence of high angle boundaries which constitute weak links through which the current density drops dramatically in the presence of a magnetic field. Critical current density also decreases strongly with the angle between transport current and the strongly superconducting a-b planes. Thus, for these materials to be utilized in large scale practical applications, the entire specimen should consist of an oriented domain with its associated quasi-single crystalline characteristics.
Conventional melt growth and texturing processes have been developed to attempt to eliminate grain boundaries and exclude weak links, thereby increasing critical current density, field trapping capacity, and levitation force of high temperature superconductors. These enhanced properties are only exhibited within a single oriented domain less than 10 mm in size. Unfortunately, because nucleation is not controlled in conventional melt growth and texturing processes, the bulk samples formed from those processes generally consist of multiple misoriented domains with high-angle domain boundaries which are also limited in size. Although the Jc within an oriented domain is generally on the order of 105 A/cm2, it decreases to values in the range of 102-103 A/cm2 when the current is required to flow through high-angle domain boundaries. To date, limitations in seed size, growth rate, and temperature processing range of high temperature superconductors have limited the sizes of single domains which can be produced by specialized processes such as seeding in the presence of chemical and temperature gradients. However, these processes require long processing time and are not suitable for mass production of superconductors.
Directional solidification techniques also have been developed for producing relatively long ( less than 100 mm) single domained samples when the cross-sectional area is limited. When the cross section exceeds approximately 3 mmxc3x973 mm, however, lateral temperature gradient causes formation of a superconductor that is multi-domained and misaligned.
Seeding has been proposed as a way to control grain orientation in the REBa2Cu3Ox superconducting system. Seeding by single-crystalline SmBa2Cu3Ox and NdBa2Cu3Ox has been successful in producing YBa2Cu3Ox domains of 30 to 50 mm in size. However, at least four main problems remain to be overcome before REBa2Cu3Ox can be used in practical applications to obtain larger materials; these are 1) the size of the oriented domain, 2) weak links within the domain, 3) limitations with respect to the types of RE that can be used, and 4) impractical manufacturing time and cost. These problems would all be alleviated, to some degree, if larger seeds were available.
In conventional melt processing by small seeds of a few millimeters (mm) in diameter, the size of the REBa2Cu3Ox that can be grown is limited as a result of grain impingement between the seeded domain and the neighboring domains which result from undesirable nucleation centers. Additionally, the degree of crystal perfection decreases, whereas the variation in chemical composition increases, as the growth front moves farther away from the seed. In other words, as the domain size increases, superconducting properties degrade. This decreasing quality of large single-domained high temperature superconductors has been confirmed by x-ray rocking curve analyses where the c-axis alignment is found to be degraded as the domain size increases. (see R. L. Meng et al., Physica C 232 (1994) 337).
Limitations in sample type have also been problematic. For example, YBa2Cu3Ox can be produced by SmBa2Cu3Ox seeds because its decomposition temperature is higher than that of YBa2Cu3Ox, but NdBa2Cu3Ox cannot be obtained in the same fashion. As a result, large single domained NdBa2Cu3Ox, which has been shown to have more desirable properties than YBa2Cu3Ox, cannot be obtained at this time.
In addition to the quality of the superconductors, the time and cost of manufacturing is also of concern. To obtain as large a domain as possible using small seeds, the cooling rate has to be extremely slow such that the manufacturing time of 20 mm size samples is typically well in excess of 100 hours. For samples on the order of 100 mm, the processing time would be measured in terms of weeks, if it were at all possible to fabricate samples of such dimensions.
To obtain large domains from small seeds, the growth front of the seeds must remain stable while proceeding for a large distance. High supercooling along the growth direction often leads to disorderly production of nuclei at the later stage of crystal growth resulting in multi-domained high temperature superconductors. To date, only when special modifications such as composition and/or sizable temperature gradients are employed can large single-domained REBa2Cu3Ox be obtained. Although the domain size may be increased by these processes, the Jc of such samples still is considerably smaller than that of small-domained specimens, indicating that weak links are still present in the apparent single-domained superconductors.
For further background information, please refer to the following publications:
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2. P. J. McGinn, W. Chen, N. Zhu, M. Lanagan and U. Balachandran, Appl. Phys. Lett. 57 (1990) 1445.
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6. D. F. Lee, C. S. Partsinevelos, R. G. Presswood, Jr. and K. Salama, J. Appl. Phys. 76 (1994) 603.
7. R. L. Meng, L. Gao, P. Gautier-Picard, D. Ramirez, Y. Y. Sun and C. W. Chu, Physica C 232 (1994) 337.
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10. D. P. Norton, A. Goyal, J. D. Budai, D. K. Christen, D. M. Kroeger, E. D. Specht, Q. He, B. Saffian, M. Paranthaman, C. E. Klabunde, D. F. Lee, B. C. Sales and F. A. List, Science 274 (1996) 755.
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13. X. D. Wu, S. R. Foltyn, P. N. Arendt, W. R. Blumenthal, I. H. Campbell, J. D. Cotton, J. Y. Coulter, W. L. Hults, M. P. Maley, H. F. Safar and J. L. Smith, Appl. Phys. Lett 67 (1995) 2397.
Accordingly, a primary object of the present invention is to solve the above problems and to provide multi-domained bulk high temperature superconductor material having strongly-linked, low-angle domain boundaries and which resembles a quasi-single domained material and a process for making the same. Further and other objects of the present invention will become apparent from the description contained herein.
The terms xe2x80x9cprocessxe2x80x9d, xe2x80x9cmethodxe2x80x9d, and xe2x80x9ctechniquexe2x80x9d are used interchangeably herein.
The designation xe2x80x9c(RE1)Ba2Cu3Oxxe2x80x9d used herein refers to REBa2Cu3Ox seed material in the form of single crystals, single-domained melt-textured pieces, and the designation xe2x80x9c(RE2)Ba2Cu3Oxxe2x80x9d refers to a final bulk superconductor precursor material to be grown as a product.
We have developed a processing method that solves the aforementioned problems by producing multi-domained bulk (RE2)Ba2Cu3Ox with low-angle domain boundaries. Since the domain boundary angles are engineered to be low and are relatively free of insulating secondary phases, these oriented domains are strongly-linked, and the bulk superconductor resembles a quasi-single domained material.
According to the method of this invention, multiple seeds, which can be small single crystals, single domained melt-textured (RE1)Ba2Cu3Ox pieces, textured substrates comprised of grains with low misorientation angles, or thick film (RE1)Ba2Cu3Ox deposited on such textured substrate, such seeds being tailored for various (RE2)Ba2Cu3Ox compounds, are arranged on (RE2)Ba2Cu3Ox superconductor precursor material in specific patterns and relative seed orientations. The superconductor precursor material may rest on a porous substrate before seeding so as to reduce the amount of liquid phase in the melt.
According to a method of this invention, seeded growth proceeds from individual seeds resulting in strongly-linked low-angle domain boundaries that contain minimal amount of insulating phases. Because seeds can be arranged in virtually any pattern, high quality (RE2)Ba2Cu3Ox elements of virtually unlimited size and complex geometry can be fabricated for use in current lead components in conventional superconducting magnets, magnetic bearings, for use in flywheel energy storage devices, as well as other applications. Additionally, the methods of the present invention will drastically reduce processing times and cost per sample and can readily be used for large scale production.
In one aspect, the present invention constitutes a novel seeding and growth process for preparing multi-domained bulk high temperature superconductors having strongly linked, low angle domain boundaries comprising the steps of:
preparing bulk (RE2)Ba2Cu3xsuperconductor precursor material;
fabricating seed material of an appropriate size for the final bulk (RE2)Ba2Cu3Ox material to be produced;
forming an assembly comprised of said seed material and said superconductor precursor material by placing said seed material in intimate contact with said superconductor precursor material in an arrangement whereby the relative misorientation between individual seed crystals is less than 15xc2x0, preferably less than 5xc2x0, and whereby the subsequent growth fronts of adjacent domains will impinge along the direction normal to the (100) or (010) planes, i.e., the [100] or [010] central axes of neighboring seeds shall be as co-linear as possible such that the resultant domain boundaries are parallel to the (100) or (010) planes;
heating the assembly to a temperature higher than the decomposition temperature of the bulk superconductor precursor material but lower than the decomposition temperature of the seed/s material for a sufficient time to decompose the precursor material;
gradually cooling the assembly to a temperature at which the bulk precursor material has solidified completely;
further cooling the assembly to room temperature; and
treating said assembly in an oxidizing atmosphere to add oxygen to the assembly and to obtain a bulk superconductor of (RE2)Ba2Cu3Ox.
In another aspect, the present invention constitutes a process for producing a bulk superconductor with a porous substrate for liquid phase removal comprising the steps of:
preparing bulk (RE2)2Ba2Cu3Ox superconductor precursor material;
placing the bulk (RE2)Ba2Cu3Ox superconductor precursor on a nonreactive, nonnucleating porous substrate comprised of (RE2)2Ba2CuO5 material;
fabricating seed material of an appropriate size for the final bulk (RE2)Ba2Cu3Ox material to be produced;
forming an assembly comprised of said seed material and said superconductor precursor material by placing said seed material in intimate contact with said superconductor precursor material in an arrangement whereby the relative misorientation between individual seed crystals is less than 15xc2x0, preferably less than 5xc2x0, and whereby the subsequent growth fronts of adjacent domains will impinge along the direction normal to the (100) or (010) planes, i.e., the [100] or [010] central axes of neighboring seeds shall be as co-linear as possible such that the resultant domain boundaries are parallel to the (100) or (010) planes;
heating the assembly to a temperature higher than the decomposition temperature of the bulk superconductor precursor material but lower than the decomposition temperature of the seed/s material for a sufficient time to decompose the precursor material;
gradually cooling the assembly to a temperature at which the bulk precursor material has solidified completely;
further cooling the assembly to room temperature;
treating said assembly in an oxidizing atmosphere to add oxygen to the assembly; and
to obtain a bulk superconductor of (RE2)Ba2Cu3Ox.
In yet another aspect, there is provided a novel sequential seeding and growth process for preparing multi-domained bulk high temperature superconductors having strongly linked, low angle domain boundaries.
In yet another aspect, there is provided a multi-domained, bulk oxide superconductor in the form of a complex oxide of RE, Ba and Cu, where RE represents at least one element selected from the group consisting of Y, Sm, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu, said superconductor having low-angle domain boundaries such that said superconductor functions as a quasi-single domained material.